1. Field of the Invention
The present invention relates to liquid crystal displays. More particularly it relates to liquid crystal displays, and its fabricating method, having improved bonding of an upper plate to a lower plate.
2. Discussion of the Related Art
Generally, a liquid crystal display (LCD) controls the light transmissivity of liquid crystal cells arranged in a matrix pattern in response to video signals so as to display a picture that corresponds to the video signals. To this end, the LCD includes a liquid crystal display panel having active matrix liquid crystal cells and driving integrated circuits (IC's) for driving the liquid crystal cells.
The driving IC's are usually manufactured in chip form. They are then mounted on a tape carrier package (TCP) film that is attached to the outer periphery of the liquid crystal panel. The driving chips are then connected to the liquid crystal display by a tape automated bonding (TAB) system of a chips on glass (COG) system. The driving IC's electrically connect to pads along edges of the liquid crystal panel. The pads electrically connect to electrode lines that connect to the liquid crystal cells. Thus, driving signals from the driving IC's are applied to the liquid crystal cells.
FIG. 1 is a plan view showing a conventional liquid crystal display panel. As shown, the liquid crystal panel 2 has a lower plate 4 and an upper plate 6. Those plates are bonded together in parallel. The liquid crystal panel 2 includes a picture display part 8 having liquid crystal cells that are arranged in a matrix pattern, and gate pads 12 and data pads 14 that connect to the driving IC's (which are not shown). Gate links 34 and data links 16, respectively, connect the gate pads 12 and the data pads 14 to the picture display part 8. A seal 10 is provided at the outer circumference of the picture display part 8. The seal assists the bonding of the lower plate 4 to the upper plate 6.
In the picture display part 8, a plurality of data lines 13 receive video signals applied via the data pads 14 and the data links 16. The data lines are arranged in such a manner as to intersect with a plurality of gate lines 11. The gate lines 11 receive scanning signals that are input via the gate pads 12 and the gate links 34. At the intersections are thin film transistors (TFTs) that switch data signals to pixel electrodes so as to drive the liquid crystal cells. The upper plate 6 has red, green and blue color filters formed separately using a black matrix, and a common transparent electrode that is formed on the color filters. The lower plate 4 and the upper plate 6 are separated by a uniform distance using a spacer to provide a cell gap.
The lower plate 4 and the upper plate 6 are bonded together using a sealant that is coated on the seal 10. The gap between the upper plate 6 and the lower plate 4 is injected-filled with a liquid crystal material and then sealed. The gate pads 12 and the data pads 14 are located along edges of the lower plate 4 that are not overlapped-by the upper plate 6. Each gate pad 12 applies a scanning signal, via a wire within the TCP film, from a gate driving IC, via a gate link 34, to a gate line 11 of the picture display part 8. Each of the data pads 14 applies video data signals from a data driving IC, via a data link 16, to a data line 13 of the picture display part 8.
In the liquid crystal panel 2, a protective film that protects the metal electrode lines and the thin film transistors is coated over the lower plate 4. The pixel electrodes connect, via contact holes, to the thin film transistors. Each pixel electrode is a transparent electrode, beneficially made from indium tin oxide (ITO), which has a relatively strong affinity to the liquid crystal material. Generally, the protective film is an inorganic material, such as SiNx or SiOx. Since inorganic films usually have high dielectric constants, conventional liquid crystal panels with inorganic protective films have a problem in that coupling between pixel electrodes and data lines 13 caused by a parasitic capacitor is high. To minimize that coupling it is often necessary to widely space the electrodes, for example, by 3 to 5 μm, such that the pixel electrodes do not overlap the data line 13. Accordingly, in conventional liquid crystal display panels it is necessary to form narrow pixel electrodes. Thus, an aperture ratio of the liquid crystal cell that depends on the area of the pixel electrode is inevitably lowered. To address this problem, an organic material, such as benzocyclobutene (BCB), spin on glass (SOG) or Acryl, etc., having a relatively low dielectric constant (about 2.7) has been used as the protective film. Such organic protective films can enable an enlarged aperture ratio.
In a high aperture ratio LCD having an organic protective film, the sealant on the seal contacts the organic protective film during the bonding of the upper plate 6 to the lower plate 4. However, many sealants, such as epoxy resins, have strong adhesion with glass but weak adhesion with respect to organic protective films. For this reason, a high aperture ratio LCD employing an organic protective film has a problem in that an impact can cause a crack between the sealant and the organic protective film that the liquid crystal material can leaked through. Additionally, an organic protective film generally has poor adhesion with the gate insulating film. Such problems are described in more detail with reference to the accompanying drawings.
FIG. 2 is an enlarged plan view of data links 16 crossing the seal 10 in FIG. 1. FIG. 2 also shows data pads 14. A semiconductor layer 18 extends from each data line into a data pad 14 that is located at an end of the data link 16. As previously discussed the seal 10 is coated with a sealant. That seal is on an organic protective film 24 (see FIG. 3A) and crosses the data links 16. Each data pad 14 contacts a transparent electrode 17 through a contact hole 19.
FIG. 3A shows a vertical section of the liquid crystal display panel taken along the A–A′ line in FIG. 2, while FIG. 3B shows a vertical section of the liquid crystal display panel taken along the B–B′ line in FIG. 2. In FIGS. 3A and 3B, the lower plate 4 has an inorganic gate insulating layer 22, semiconductor layers 18, and data links 16 on a lower glass substrate 20, and an organic protective film 24 that entirely coats the lower plate 4. The upper plate 6 has a color filter and black matrix 28 on one side of an upper glass substrate 30 and a common transparent electrode 26. The lower plate 4 and the upper plate 6 are bonded to each other using a sealant coated on the seal 10. The sealant has poor adhesion to the organic protective film 24. Also, the organic protective film 24 has poor adhesion with the gate insulating film 22. Floating of the organic protective film 24, or leakage of a liquid crystal material 32, may result from a crack generated from an exterior impact. FIG. 3B shows a picture display part 8 that holds the liquid crystal material 32.
FIG. 4 is an enlarged plan view of FIG. 1 showing the seal 10 crossing gate links 34. FIG. 4 also shows gate pads 12. The gate pads 12 contact transparent electrodes 17 through contact holes 19. As previously mentioned the seal 10 coated with a sealant.
FIG. 5A shows a vertical section of the liquid crystal display panel taken along the A–A′ line in FIG. 4, while FIG. 5B shows a vertical section of the liquid crystal display panel taken along the B–B′ line in FIG. 4. In FIGS. 5A and 5B, the lower plate 4 has gate links 34 and a gate insulating layer 22, disposed on a lower glass substrate 20, and an organic protective film 24 that is coated on the gate links 34 and on the gate insulating layer 22. The upper plate 6 has a color filter and black matrix 28 on an upper glass substrate 30, and a common transparent electrode 26. The lower plate 4 and the upper plate 6 are bonded to each other using the sealant coated on the seal 10. Again, the sealant has poor adhesion to the organic protective film 24.
As a result, a liquid crystal display panel employing an organic protective film has a problem in that, due to poor adhesion between the sealant and the organic protective film, or between the organic protective film and the gate insulating film, a crack is readily generated by an exterior impact. This can cause leakage of the liquid crystal material 32 in the picture display part 8.